Date:   Mon, 27 Feb 2023 09:42:54 +0100
From:   Roman Kagan <rkagan@amazon.de>
To:     Vincent Guittot <vincent.guittot@linaro.org>
CC:     Peter Zijlstra <peterz@infradead.org>,
        <linux-kernel@vger.kernel.org>,
        Valentin Schneider <vschneid@redhat.com>,
        Zhang Qiao <zhangqiao22@huawei.com>,
        Ben Segall <bsegall@google.com>,
        Waiman Long <longman@redhat.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mel Gorman <mgorman@suse.de>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        "Daniel Bristot de Oliveira" <bristot@redhat.com>,
        Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>
Subject: Re: [PATCH v3] sched/fair: sanitize vruntime of entity being placed
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Mail-Followup-To: Roman Kagan <rkagan@amazon.de>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Peter Zijlstra <peterz@infradead.org>, linux-kernel@vger.kernel.org,
        Valentin Schneider <vschneid@redhat.com>,
        Zhang Qiao <zhangqiao22@huawei.com>,
        Ben Segall <bsegall@google.com>, Waiman Long <longman@redhat.com>,
        Steven Rostedt <rostedt@goodmis.org>, Mel Gorman <mgorman@suse.de>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Ingo Molnar <mingo@redhat.com>, Juri Lelli <juri.lelli@redhat.com>
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On Tue, Feb 21, 2023 at 06:26:11PM +0100, Vincent Guittot wrote:
> On Tue, 21 Feb 2023 at 17:57, Roman Kagan <rkagan@amazon.de> wrote:
> > What scares me, though, is that I've got a message from the test robot
> > that this commit drammatically affected hackbench results, see the quote
> > below.  I expected the commit not to affect any benchmarks.
> >
> > Any idea what could have caused this change?
> 
> Hmm, It's most probably because se->exec_start is reset after a
> migration and the condition becomes true for newly migrated task
> whereas its vruntime should be after min_vruntime.
> 
> We have missed this condition

Makes sense to me.

But what would then be the reliable way to detect a sched_entity which
has slept for long and risks overflowing in .vruntime comparison?

Thanks,
Roman.


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